As FIG. 1 illustrates, conventional duplex communication systems 100 typically utilize a single satellite 102 for communication between a remote ground terminal 104 and a hub station 106. The satellite 102, a remote terminal antenna 108 at the remote ground terminal 104, and a hub antenna 110 at the hub station 106 form an independent forward point-to-point link (i.e., from the hub antenna 110 to the remote terminal antenna 108 via the satellite 102) and an independent return point-to-point link (i.e., from the remote terminal antenna 108 to the hub antenna 110 via the satellite 102). The hub station 106 typically uses a wideband carrier with Time Division Multiple Access (TDMA) coding to communicate with a plurality of remote ground terminals 104 via point-to-point forward links. And, the remote ground terminals 104 typically use Single Channel Per Carrier (SCPC)/Frequency-Division Multiplexing (FDM) to communicate with the hub station 106. If the bandwidths used for TDMA and SCPC/FDM overlap, interference cancellation is used to resolve the interference caused by that overlap. See, e.g., S. Jayasimha and J. Paladugula, “Canceling Echoes distorted by Satellite Transponders,” Proc. of Natl. Conf. on Comm. (NCC-2006) pp. 112-116, Omega Scientific Publishers, New Delhi, ISBN 81-85399-80-8.
The dimensions (e.g., the diameter for a parabolic antenna or the height and width for a square horn antenna) of an antenna aperture directly control the gain and the beam-width of the main and side lobes of that antenna. The hub antenna 110 is usually much larger than the remote terminal antenna 108, and therefore aperture size and gain are usually much greater for the hub antenna than for the remote terminal antenna 108. The ratio of hub antenna 110 aperture size to remote terminal antenna 108 aperture size is typically 4:1, with the diameter of the hub antenna 110 aperture typically being larger than 4 meters.
The effective aperture, Ae, of an antenna is the area of the antenna that is presented to the transmitted or received signal, which is directly related to the size of the antenna. More particularly, the effective aperture, Ae, of an antenna is related to the physical aperture, A, of the antenna according to the following relationship:Ae=Ka·A,wherein Ka is the antenna aperture efficiency and A is calculated as the physical area of the antenna (i.e., π·diameter2/4). And, the gain, G, of an antenna is related to the effective aperture, Ae, of the antenna by the following relationship:G=4πAe/λ2.wherein λ is wavelength of the transmitted or received signal. Thus, the gain of the remote terminal antenna 108 will depend on the physical aperture size, A, the antenna aperture efficiency, Ka, and the wavelength, λ, of the transmitted or received signal.
The half-power beam-width (HPBW) of an antenna is the angular separation between the half power points on the antenna radiation pattern, where gain, G, is one half the maximum value. The HPBW of an antenna is calculated in radians as:HPBW=λ/√{square root over (Ae)}.For a conventional parabolic reflector antenna in the 3 decibel (dB) bandwidth of the main lobe and a diameter of 36λ, the HPBW will be slightly less than 0.0524 radians (≈3°), which is slightly less than the typical 3° angular separation, s, of adjacent satellites. Accordingly, a remote terminal antenna 108 with those properties will receive not only signals from the satellite to which it is pointing, but also unwanted signals from adjacent satellites (i.e., Adjacent Satellite Interference (ASI)). ASI occurs in ground terminals with small aperture antennae because the antennae are too small to be able to focus properly on the specific satellite of interest (i.e., pointing errors). ASI is the especially problematic for co-polarized adjacent satellite transponders.
There are three conventional methods for reducing ASI. The first method includes increasing the size of the antenna aperture, A, so the beamwidths decrease. The second method includes aperture tapering to increase side lobe suppression. And, the third method includes using spread-spectrum methods to reduce ASI Power Spectral Density (PSD) according to the bandwidth expansion factor. Each of those methods, however, has drawbacks. For example, larger apertures are not as suitable for mobile applications where aperture size is restricted, hyperbolic aperture illuminations from aperture tapering blow up main lobe beamwidth, and spread-spectrum methods reduce transponder throughput. Accordingly, there is a need for a remote terminal with a reduced aperture antenna that provides equivalent or improved ASI over conventional remote terminal antennae, particularly for mobile applications where aperture size is restricted, such as Man-Pack and SATCOM-on-the-move applications.